A video apparatus, such as a television receiver or a computer monitor, incorporates some form of image display, which may be a cathode ray tube (CRT), for example. The CRT produces one or more electron beams via an electron gun assembly which are caused to impinge or land on a phosphor display screen by electron beam forming and accelerating voltages. The display screen emits light in proportion to the energy of the electron beam or beams.
A deflection yoke produces electromagnetic deflection fields in response to cyclically varying deflection currents that act to deflect or scan the electron beams across the display screen in a predetermined pattern to form a raster. The electron beam energy, i.e., current, is controlled in response to the information contained in a video signal in order to reproduce a video image on the display screen.
The video signal includes synchronizing information to properly synchronize the line rate or horizontal and field rate or vertical deflection of the electron beams with the video information in order to provide a stable video display. The video apparatus may incorporate circuitry that internally derives horizontal and vertical rate synchronizing (sync) pulses by frequency dividing a high frequency clock signal. The derived horizontal rate pulses of such a countdown circuit are locked to the horizontal rate sync information contained in the video signal by phase locked loop circuitry. For NTSC broadcast video signals that produce a line interlaced scan or deflection, the vertical rate sync information will occur at a predetermined ratio with respect to the horizontal rate sync information. If the vertical rate sync information of the video signal is found to occur at a "standard" ratio with respect to the horizontal rate sync information, synchronization of the vertical deflection circuitry will be accomplished via the internally generated sync pulses. If the video signal vertical rate sync information does not occur at the standard ratio, direct synchronization of the vertical deflection circuit by the video signal vertical rate sync information may be provided. This direct synchronization of the vertical deflection circuitry is appropriate for truly non-standard signal sources, such as a VCR operating in a fast search or slow motion mode, for example.
Personal computers and video game circuitry may provide video signals that produce a noninterlaced scan in which the horizontal-to-vertical sync information ratio is fixed but does not occur at the ratio attributed to a designated standard signal source. In such a situation, direct synchronization of the vertical deflection circuitry by the vertical rate sync information of the video signal will occur. The circuitry incorporated in personal computers and, in particular, video game apparatus, may lack the sophistication of those circuits found in conventional television receivers, for example. As a result, the sync signal may comprise poorly formed pulses which can cause erratic triggering and synchronization of the deflection circuits to occur. Vertical jitter of the displayed video image may result. It would be desirable in such a situation to cause the deflection circuitry to operate in the standard or countdown mode, rather than the nonstandard or direct synchronization mode.